1. Technical field
The invention relates to nuclear reactors cooled by a pressurized liquid of the type having a pressure resistant vessel with inlet and outlet pipes situated substantially at the same level, above the core, and internals having a casing supporting the core and defining with the vessel a down flow coolant path from the inlet pipes to a space situated under the core and an upper internal equipment defining a path for the coolant which flows upwardly out of the core towards the outlet pipes.
2. Prior Art
FIG. 1 is a simplified representation of internals widely used at the present time in PWRs. The reactor includes a vessel 10 closed by a cover 12 for defining a pressurized volume containing the core of the reactor. The core is formed of fuel assemblies 14, two of which are shown schematically, disposed vertically and side by side. The core of the reactor is carried by lower internals formed by a basket-shaped structure. The lower internals include a bottom plate 16 also called "core support plate", a cylindrical barrel 18 and a flange 20 bearing on an internal shoulder of the vessel. A dividing structure 21 is interposed between the periphery of the core (which has a polygonal crosssection) and the barrel 18. The upper internals located above the core include an upper core plate 22, situated just above the core, which indexes the fuel assemblies 14 and holds them in position.
The upper internals further include a support plate 24 connected to plate 22, by control cluster guide tubes 26 in the illustrated embodiment. In other cases, plates 22 and 24 are joined together by spacers. The support plate 24 is fixed to the vessel; as illustrated, plate 24 and flange 20 are clamped between vessel 10 and cover 12.
The cluster guides 26 are situated above fuel assemblies 14 constructed to receive control clusters (not shown) movable vertically by shafts which pass through the cover through penetration sleeves 28.
The lower internal equipments direct the pressurized water which enters the vessel through inlet pipes 30 down into the annular space between vessel 10 and barrel 18. The pressurized water thus arrives in the space situated below plate 16. It flows into the fuel assemblies 14 of the core through openings 32 in the bottom plate, upwardly through the core radially inwardly of the partitioning 21 and through the upper core plate 22 to reach the upper internals. The cluster guides 26 protect the control clusters, when they are raised, against forces induced by the water flow leaving the core. The water flow is initially vertically upwards, then deflected within the upper internals and it leaves barrel 18 through outlet nozzles 34 placed in alignment with outlet pipes 36 carried by the vessel. The outlet nozzles 34 are fitted to pipes 36 so that a gap exists when the reactor is cold for inserting or withdrawing the lower internals into and out of the vessel. Differential thermal expansion between the carbon steel vessel 10 and the stainless steel barrel 18 closes the gap and reduces the flow by-passing the core to a very low value when the reactor is at its operating temperature.
The lifting forces exerted on the fuel assemblies by the rising flow of pressurized water in the core are taken by the upper core plate 22 which transfers them to the support plate 24 in abutment against cover 12.
The arrangement which has just been described is well known and widely used. It has the drawback that the cluster guides 26, and especially those which are adjacent the outlet nozzles 34, are subjected, over substantially the whole of their length, to a radially deflected pressurized water flow and so to high forces. The transverse flow may cause vibratory movements of the cluster guides, and the transverse flow through the guide network is accompanied by high pressure losses.
The above drawbacks are all the more serious when a large number of clusters and of guides 26 are used and when the distribution pitch of the fuel assemblies is tight. For better use of the fissile material, future reactors will have, in addition to the control clusters, clusters having other functions, for example varying the neutron energy spectrum. The substitution contemplated of the present square shaped assemblies with assemblies distributed in a triangular pattern, having a tighter array of fuel rods, is likely to further increase the number of cluster guides required.
Different solutions may be envisaged for attenuating the above drawbacks, using several different approaches. A possible approach consists in locating a perforated shroud between the barrel 18 and the set of cluster guides. The water flow towards the annular space between the shroud and the casing provided with the nozzles is then distributed among all the holes and the maximum speed is reduced between the guides. That arrangement (European Pat. No. 125,325) increases the diameter of the vessel. In addition, should a break in the primary circuit occur, the pressure loss which the flow undergoes on passing through the restriction represented by openings in the shroud opposes the pressure balancing and increases the mechanical stresses.
Another possible approach consists in rendering the flow vertical in the zone occupied by the cluster guides. Different modifications of the arrangement of FIG. 1 allow this result to be attained. A first solution (U.S. Pat. No. 3,366,546) consists in placing, around the set of cluster guides, a shroud which is formed with holes in its upper part only. Again, the arrangement requires increasing the diameter of the vessel. A further solution consists in placing the outlet pipes of the vessel above the inlet pipes and in placing, between the two sets of pipes, a cylindrical ring connecting the casing to the vessel. The outlet pipes may then be above the level of the guides so that flow deflection takes place above the latter. This arrangement requires lengthening the vessel of the reactor. In addition, it makes it more difficult to "flood" the core again upon rupture of the hot leg of the primary circuit (i.e. the leg connected to the outlet pipe) by injecting water arriving through the inlet pipe, due to the high loss of motive power.